Data networks allow communication between a plurality of users by networking, i.e. connecting the individual users together. Communication thereby means the transmission of data between the users. The data to be transmitted is thereby sent in the form of data telegrams, i.e. the data is packed together in packets and sent in this form via the data network to the corresponding recipient. The term data packets is therefore also used. The term “data transmission” is totally synonymous in this document with the above-mentioned transmission of data telegrams or data packets. Networking itself is for example achieved in the case of switchable high-speed data networks, in particular the Ethernet, by inserting at least one switching matrix between two users, which is connected to both users. Each switching matrix can be connected to more than two users. Each user is connected to at least one switching matrix but not directly to another user. Users are for example computers, stored-program controllers (SPS, PLC) or further components, which exchange and in particular process electronic data with other components. Switching matrices are for example what are known as switches, hubs, bridges, routers, etc. Unlike bus systems, in which every user can access any other user of the data network directly via the data bus, switchable data networks are exclusively point-to-point connections, i.e. a user can only access all other users of the switchable network by forwarding the data to be transmitted by means of one or a plurality of switching matrices. The data packets in a switchable data network are also referred to as frames, as the useful data transmitted is generally encapsulated in a frame, as special synchronization and error-recognition bits are inserted before and after the useful data and frame it in this way.
In distributed automation systems, for example in the field of drive engineering, specific data has to reach the users for which it is intended and be processed by the recipients at specific times. This is referred to as realtime-critical data or data traffic, as failure of the data to arrive at its destination at the correct time has undesirable results for the user.
Time multiplex methods for transmitting data via broadband networks, in which time slots are reserved for different transmission channels and transmission channels are allocated to users on a temporal basis are known from the internet document by Prof. Dr. K. Irmscher, University Leipzig, “B-ISDN/ATM, Standardisiertes Breitbandnetz, ATM-Asynchronous Transfer Mode”. With what is known as synchronous time multiplexing (Synchronous Time Division=STD) transmission frames are defined, which comprise a specific number of time slots of fixed size. Each user is assigned a specific time slot within the transmission frame, during which said user can send and receive. A transmission channel is therefore identified by the position of the time slot within the transmission frame.